Various apparatus and methods are known for generating heat for contribution to water housed within a water heater tank by combustion of a mixture of fuel and air. In an example of such arrangement, a gas burner is disposed below the water tank in communication with a source of fuel and a source of air and an igniter so that, upon actuation of the igniter, the gas/air mixture combusts at the burner surface within an area, or burner box or combustion chamber, below the tank. The combustion heats the volume below the tank, allowing heat transfer through the tank floor into the tank's interior volume and the water contained therein. In addition, a flue pipe extends from the burner box up and through the tank volume to receive and allow passage of combustion exhaust gas through the tank. United States published patent application 2011/0214621, the entire disclosure of which is incorporated by reference herein for all purposes, discloses the use of a baffle disposed within the central flue pipe to facilitate transfer of heat from the exhaust gas to the flue pipe wall and, thereby, to water within the tank that surrounds the flue pipe. Rising through the flue pipe, the exhaust gas passes from the pipe at its top into a manifold disposed outside and above the tank. From the manifold, the gas passes into a secondary tube that extends back down through the top of the tank wall and back into the tank interior volume in parallel with the first flue pipe. Near the bottom of the inner tank volume, the tube turns and commences a coil around the primary flue pipe, thereby increasing the surface area of the secondary tube within the interior volume and correspondingly increasing the transfer of heat from gas flowing through the tube to the tube wall and thence to the water in the tank surrounding the tube. From the coil, the secondary tube extends outward through a side wall of the tank to a blower, which creates negative pressure to thereby draw the exhaust gas from the manifold, through the secondary tube, and out of the tank to a vent system. In another embodiment, the secondary tube is connected to the primary tube within the interior tank volume, before the primary tube extends through the top wall.
U.S. Pat. No. 7,559,293 discloses a gas-fired water heater with a down-fired combustion system comprised of a blower and a down-fired burner. The blower receives a mixture of fuel and air and pushes the mixture to a cylindrically-shaped burner that extends vertically downward into a vertical pipe that extends through the center of the water heater's interior volume. The center pipe extends down through the tank interior volume and the tank bottom wall to a collector. When an igniter is actuated, the fuel/air mixture ignites and combusts at the burner surface. The blower pushes exhaust gas vertically downward through the center pipe, allowing heat transfer from the exhaust gas through the center pipe wall and to water in the tank. The exhaust gas collects in the bottom collector and then rises through a plurality of pipes extending upward from the lower collector, through the bottom tank wall, through the top tank wall and to a second, upper, collector. From the upper collector, the exhaust gas passes back down through a second plurality of pipes extending through the upper tank wall, through the tank interior volume, through the bottom tank wall, and to a third, lower collector, from which the exhaust gas is vented from the system.